Cisco Router unwanted dialups

Can anyone help me with the configuration of a Cisco 760 ISDN Router.

The problem is that the router keeps bringing up the isdn line for no apparent reason resulting in a horrendous phone bill for the client.

The dial ups seam to be random andI guess it is a network issue (being caused by people browsing the network through network neighborhood).

All the pc’s are running a variety of Win2k Win98 and WinME can anybody suggest a starting point to try and trace the cause of the dial ups.